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Previous Article | Next Article
The Journal of Neuroscience, July 15, 2000, 20(14):5437-5448
Intrinsic and Extrinsic Contributions to Auditory Selectivity in a Song Nucleus Critical for Vocal Plasticity
Merri J. Rosen and Richard Mooney Department of Neurobiology, Duke University Medical Center, Durham, North Carolina 27710
ABSTRACT
TOP
ABSTRACT
INTRODUCTION
The development, maintenance, and perception of learned vocalizations in songbirds are likely to require auditory neurons that respond selectively to song. Neurons with song-selective responses have been described in several brain nuclei critical to singing, but the mechanisms by which such response properties arise, are modified, and propagate are poorly understood. The lateral magnocellular nucleus of the anterior neostriatum (LMAN) is the output of an anterior forebrain pathway (AFP) essential for learning and maintenance of song, processes dependent on auditory feedback. Although neurons throughout this pathway respond selectively to auditory presentation of the bird's own song, LMAN is the last stage at which responses to this auditory information could be transformed before being transmitted to vocal motor areas, where such responses may influence vocal production. Indeed, previous extracellular studies have indicated that LMAN's auditory selectivity is greater than that at earlier stages of the AFP. To determine whether LMAN local circuitry transforms or simply relays song-related auditory information to vocal control neurons, it is essential to distinguish local from extrinsic contributions to LMAN's auditory selectivity. In vivo intracellular recordings from LMAN projection neurons, coupled with local circuit inactivation, reveal that much of LMAN's song selectivity is supplied by its extrinsic inputs, but selective blockade of GABA receptors indicates that local inhibition is required for the expression of song selectivity. Therefore, LMAN neurons receive highly song-selective information, but LMAN's local circuitry can mask these selective inputs, providing a mechanism for context-dependent auditory feedback.
Key words: auditory selectivity; song selectivity; birdsong; zebra finch; LMAN; in vivo; intracellular; local circuitry; inactivation; GABA; bicuculline
INTRODUCTION
Auditory responses selective for communication sounds exist in neurons of many vertebrates, including primates, bats, frogs, and songbirds (Narins and Capranica, 1976
; Margoliash, 1983
The neural substrate for singing comprises a vocal motor pathway (VMP) used throughout life and an anterior forebrain pathway (AFP) needed for juvenile song development and, in some species, auditory maintenance of adult song (Fig. 1A) (Nottebohm et al., 1976
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Figure 1. Schematics of the song circuit and LMAN. A, The VMP (white) used in singing includes HVc, RA, nAM, the vocal motoneurons in nXIIts, and respiratory premotor neurons in nAM and nRAm. The AFP (gray) used in auditory-dependent song development as well as in adult song maintenance includes area X, DLM, and LMAN. Auditory areas presynaptic to HVc include Ov, field L, and NIf (dotted); the dashed line indicates abbreviated ascending auditory structures. B, The diagram of LMAN's intrinsic and extrinsic connections shows the experimental protocol. LMAN contains projection neurons (PN) that innervate area X and RA and GABAergic interneurons (IN) that innervate these projection neurons; both cell classes receive excitatory input from DLM and other LMAN projection neurons. Intracellular recordings were made from LMAN projection neurons while delivering auditory stimuli ("song"). In some experiments, GABAA receptor antagonists or GABA were applied to LMAN via a puffer pipette positioned immediately dorsal to the nucleus. DLM, Dorsolateral part of the medial thalamus; HVc (used as a proper name); L, field L; LMAN, lateral magnocellular nucleus of the anterior neostriatum; nAM, nucleus ambigualis; NIf, nucleus interfacialis; nRAm, nucleus retroambigualis; nXIIts, tracheosyringeal portion of the hypoglossal nucleus; Ov, nucleus ovoidalis; RA, robust nucleus of the archistriatum; X, area X.
LMAN's auditory nature is of special interest because this nucleus is strongly implicated in auditory processes essential to the development, adult maintenance, and perception of song (Price, 1979
MATERIALS AND METHODS
Subjects. Experiments used adult [95-440 posthatch days (PHD)] male zebra finches (Taeniopygia guttata) in accordance with a protocol approved by the Duke University Institutional Animal Care and Use Committee. Birds were bred and raised in our colony, housed with their parents until 40 PHD, and then moved into holding cages of six to eight birds.
Stimuli. Before the experiment, song was recorded from birds placed in a small recording chamber (Industrial Acoustics, Bronx, NY) with an adult female to induce them to sing. Songs were recorded and edited with LabView software (National Instruments, Austin, TX; all custom software for this study was written by M. Rosen, F. Livingston, R. Neumann, and R. Balu). Amplified vocalizations from a microphone were low-pass filtered at 10 kHz, digitized at 20 kHz (National Instruments data acquisition board AT-MIO-16E2), and stored on a personal computer (PC). Edited songs included either one or two exemplary motifs. Presented stimuli always included the BOS, reversed-syllable BOS (e.g., syllables ABCD presented as DCBA, perturbing global but not local temporal information), and reversed BOS (i.e., song played backward, perturbing local and global temporal order, while maintaining spectral information). Conspecific (another adult male) and heterospecific (male Bengalese finch) song and white noise (a Gaussian-distributed pseudorandom sequence) often were also presented. Stimuli were 1-3 sec in duration.
Preparatory surgery. Two days before electrophysiological recording, birds were food and water deprived for 1 hr, injected intramuscularly with equithesin (2 mg/kg, i.m.: 0.85 gm of chloral hydrate, 0.21 gm of pentobarbitol, 0.42 gm of MgSO4, 2.2 ml of 100% ethanol, and 8.6 ml of propylene glycol brought to a 20 ml final volume with dH2O), and placed in a stereotaxic device (45° head angle; H. Adams, California Institute of Technology). Lidocaine (2%; 20 µl) was injected subcutaneously, and the scalp was dissected along the midline. LMAN's location was marked using stereotaxic coordinates (5.15 mm rostral and 1.75 mm lateral from the bifurcation of the midsagittal sinus). A stainless steel post was mounted to the caudal skull with dental cement, the wound was closed with cyanoacrylate, antibiotic was applied, and the bird was kept warm (33°C) until recovery (2-4 hr).
In vivo electrophysiology, song presentation, and drug application. Immediately before electrophysiological recording, birds were injected intramuscularly with 20% urethane in dH2O (75-100 µl total; Sigma, St. Louis, MO) in 25 µl doses at 30 min intervals. Urethane does not suppress neural activity and has been used in previous studies for extracellular characterization of song selectivity in LMAN (Doupe and Konishi, 1991
Sharp electrodes (borosilicate glass, BF100-50-10; Sutter Instrument, Novato, CA) were pulled to 60-125 M
when filled with 3 M K-acetate (or in some cases 3 M Cs-acetate), 5% neurobiotin (for histological reconstruction), and in some cases 100 mM QX-314 (a Na+ channel blocker; Research Biochemicals, Natick, MA). A hydraulic microdrive (Soma Scientific, Irvine, CA) was used to lower electrodes to the nucleus (~2000 µm). An AxoClamp 2B intracellular amplifier (Axon Instruments, Foster City, CA) used in bridge mode recorded intracellular potentials, which were low-pass filtered at 3 kHz, digitized at 10 kHz, and stored on a PC. LMAN neurons were identified on-line by their characteristic spike shape and firing patterns (see Livingston and Mooney, 1997
60 mV and robust spontaneous synaptic activity was present. In some cases, input resistances and responses to current pulse injections were collected; both instantaneous and mean (over the 1 sec current pulse) firing rates were calculated in response to depolarizing currents. All values are reported as the mean ± SEM.
Ten to thirty iterations of each auditory stimulus, delivered at intervals ranging from 7 to 11 sec, were presented at ~70 dB (rms; A-weighting) through a speaker 20 cm behind the bird. Peristimulus time histograms (PSTHs; 25 msec bin width) and median-filtered averaged membrane potential traces (see below) were computed on-line to allow immediate assessment of the responsiveness to the various auditory stimuli and to aid in experimental decisions. As noted in the results for certain cells, tonic negative or positive currents were injected through the recording electrode to shift the resting membrane potential of the cell.
For pharmacological experiments, a second micropipette [10-20 µm tip; filled with 250 mM GABA (Research Biochemicals) or 5 mM bicuculline methiodide (Sigma) in 0.9% saline] was lowered through a second craniotomy to a point slightly dorsolateral to LMAN. Drugs were pressure-ejected with a Picospritzer (General Valve, Fairfield, NJ) in 10-200 msec pulses at 40 psi. For bicuculline treatment, which took 30-90 min to wash out, the pressure pipette was removed after drug application. After bicuculline application, LMAN neurons exhibited rhythmic bursting behavior that was used to assess the drug's effect in subsequent recordings (see Fig. 6). GABA was applied during interstimulus intervals while recording, because it washed out very quickly (10-120 sec). Effectiveness of the GABA inactivation was assessed during and after the application period by monitoring changes in spontaneous PSP amplitude, as well as by monitoring the ability of a positive current pulse to evoke action potentials.
Data analysis. The suprathreshold responsiveness (RFiringRate) of cells with spiking activity was calculated by RFiringRate = SFR
To compare suprathreshold and subthreshold responses, response strengths were expressed as z-scores. The suprathreshold z-score (ZFiringRate) is given by the difference between the average firing rate during stimulus presentation and that during a 1.0-1.5 sec baseline period before stimulus presentation, divided by the SD of this difference:
where
FR is the mean firing rate during the stimulus,
FR is the mean firing rate during the baseline period, and the denominator is the SD of SFR
To quantify each neuron's response strength, the psychophysical measure d' (Green and Swets, 1966
where d'FiringRate represents suprathreshold responsiveness and d'area represents subthreshold responsiveness.
is the mean value of R (as described above), and
2 is its variance. This measure of selectivity takes into account both the mean and the variance of a cell's responses. A d' value of 0.7 was used as the criterion for identifying a cell as "song-selective," that is, responding more to forward than to reversed BOS; this corresponded to a significance level of p = 0.036 as measured by a paired t test comparing Rarea or RFiringRate values for 20 presentations of BOS and versus reversed BOS.
For non-BOS stimuli, significance was determined with paired t tests comparing baseline and stimulus responses at suprathreshold and subthreshold levels (z-scores); this detected both "excitatory" and "inhibitory" responses.
Histology. Cells were stained with neurobiotin using positive currents (750 pA; 500 msec at 1 Hz). After the recording session, birds were deeply anesthetized with equithesin and transcardially perfused with 0.9% saline for <5 min, followed by 4% paraformaldehyde (PFA) in 25 mM sodium phosphate buffer for ~30 min. Brains were removed and post-fixed in 4% PFA with 30% sucrose overnight, blocked sagittally, and sectioned on a freezing microtome at 60 µm. Sections were processed using standard techniques (see Kittelberger and Mooney, 1999
RESULTS
Sharp electrode intracellular recordings were made from 299 LMAN neurons in 47 adult male zebra finches (Fig. 1B). LMAN neurons were identified by their spike shapes and responses to depolarizing currents (Fig. 2B) (Livingston and Mooney, 1997
0.7). The intrinsic and morphological properties are first described for a subset of these neurons.
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Figure 2. Morphological and physiological characterization reveals that song-selective LMAN neurons are projection neurons. A, Camera lucida reconstructions (objective magnification, 63×, left, and 10×, right) of two song-selective LMAN neurons show that these cells are spiny projection neurons with bifurcated axons projecting toward RA and area X and local collaterals within LMAN. Because of the great distance between LMAN and RA (>4 mm), axons were rarely completely filled to RA. Reconstructions are in the sagittal plane; rostral is to the left; dorsal is up. B, Five traces (top) are the responses of a song-selective LMAN projection neuron to injected currents (bottom). Note the high spontaneous synaptic activity and gradual spike onset and sharp afterhyperpolarization characteristic of these neurons. C, The highly linear relationship of firing frequency to current injection amplitude (n = 49 cells) is shown. D, Instantaneous firing frequency is plotted as a function of spike interval number, showing slight accommodation (1 sec pulses; +600 and +800 pA; n = 49 cells; dotted lines are linear fits for each current amplitude). E, Examples of a LMAN projection neuron's selective response to BOS versus reversed BOS are shown. The 10 traces at the top are raw intracellular current-clamp records of responses to repeated song stimuli [depicted as an oscillogram in the lowest trace, with syllables delineated by letters a-d (i, introductory notes)]. The plot immediately below the raw traces is an action potential PSTH (25 msec bin width), below which is the median-filtered average Vm showing underlying subthreshold responses (see Materials and Methods).
Song-selective LMAN neurons: morphology and intrinsic properties
Song-selective LMAN neurons had spinous dendrites and bifurcating axons projecting ventrally to area X and caudally toward RA (Fig. 2A) and were morphologically indistinguishable from LMAN projection neurons studied in vitro (Livingston and Mooney, 1997
Song-selective properties
Song-selective responses typical of LMAN projection neurons are shown in Figure 2E. Individual current-clamp records (top) showing suprathreshold and subthreshold responses to forward and reversed BOS playback were used to construct PSTHs and median-filtered trace averages (bottom). BOS playback consistently elicited subthreshold depolarizations accompanied by volleys of action potentials, whereas reversed BOS playback elicited no consistent subthreshold or suprathreshold responses. Of 299 cells tested, 183 (78%) displayed selective responses similar to this example, whereas 52 were nonselective (d' < 0.7). Another subset of LMAN projection neurons did not fire action potentials spontaneously or to BOS playback, despite displaying robust spontaneous synaptic activity (n = 64). However, 53 (83%) of these "silent" cells still displayed subthreshold selectivity (Materials and Methods), a percentage similar to that of selective spiking neurons. As described in extracellular studies (Doupe, 1997
Responses to non-BOS stimuli
Most cells responded only to BOS, but weaker subthreshold and/or suprathreshold responses to other stimuli were occasionally seen (Fig. 3) and included both excitatory and inhibitory responses (see Materials and Methods). Reversed-order BOS frequently elicited excitatory responses (81/236 cells), whereas inhibition was elicited in only 1 cell. Responses to reversed BOS were extremely rare (excitatory, 3/236 cells; inhibitory, 13/236 cells), and excitation consisted only of an onset response. Very few cells responded to conspecific (excitatory, 8/106; inhibitory, 4/106) or heterospecific (excitatory, 5/106; inhibitory, 5/106) songs, whereas white noise elicited more inhibitory (10/35) than excitatory (2/35) responses. The suprathreshold components of these response properties are similar to those described in extracellular studies of LMAN (Doupe, 1997
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Figure 3. Song-selective LMAN neurons rarely exhibited responses to non-BOS auditory stimuli. A, Proportion of BOS-responsive cells (n = 236; includes suprathreshold and/or subthreshold responsivity) that also responded to other auditory stimuli. Vertical dashed lines delineate subsets of cells that also received other stimuli. All 236 cells received reversed order BOS and reversed BOS, 106 of these cells were also tested with conspecific (con) and heterospecific (het) song, and 35 of these cells were also tested with white noise. BOS responses were always excitatory; inhibitory responses occurred to other stimuli, especially white noise. B1-B4, Examples of the BOS and non-BOS responses in cells from four birds. Significant (by paired t test or d'; see Materials and Methods) excitatory and inhibitory responses to auditory stimuli are indicated by + and
Song selectivity: local circuit contributions
In vitro studies have shown afferents to LMAN from DLM to be excitatory, whereas inhibitory input arises locally from LMAN interneurons (Livingston and Mooney, 1997
Comparison of subthreshold and suprathreshold selectivity
The intracellular approach allows a within-cell comparison of the selectivity of synaptic inputs with suprathreshold output. In LMAN, greater suprathreshold than subthreshold selectivity could be evidence of local refinement at a single-cell level, for example, by a nonlinear thresholding mechanism (Jagadeesh et al., 1997
One impression was that certain cells displayed large d' values at a suprathreshold level, with minimal subthreshold change, presumably because their resting membrane potentials were closer to spike threshold, resulting in small-amplitude EPSPs. Indeed, plots of firing rate or area versus resting membrane potential revealed a slight negative correlation between subthreshold selectivity and Vrest (Fig. 4C,D; r =
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Figure 4. A population analysis of LMAN projection neurons reveals equivalent song selectivity at subthreshold and suprathreshold levels. A, Normalized responses (z-scores) to BOS are plotted against those to reversed BOS, for both suprathreshold (firing rate) and subthreshold (area) activity (n = 299 cells). The points to the right of the diagonal line are cells with a bias for forward song. B, The discrepancy between subthreshold and suprathreshold selectivity for all spiking cells (n = 235) is plotted as a cumulative proportion of d' values. The dashed vertical line indicates the criterion for significant selectivity (d' = 0.7). C, D, Scatter plots of suprathreshold and subthreshold d' values with resting Vm show a significant negative correlation for area (r =
Current injection to detect local inhibition
In the song nucleus HVc and the mammalian visual cortex, selectivity involves pronounced inhibition and excitation (Ferster, 1986Vm = +52 mV; n = 8 cells), rendering spontaneous IPSPs hyperpolarizing (Fig. 5A, arrows) and distinct from EPSPs, which remained depolarizing. In this tonically depolarized state, selectivity across these cells (although not for this example) was significantly reduced but still robust (d'rest = 1.57 ± 0.22; d'depol = 1.05 ± 0.16; p = 0.04). This is consistent with the population data in which more positive cells exhibited reduced subthreshold selectivity. Importantly, no component of the subthreshold response to forward or reversed BOS was hyperpolarizing (Fig. 5A). This suggests that the song-evoked synaptic drive to LMAN does not involve appreciable local GABAergic inhibition.
Although song-evoked inhibition was not detected, LMAN has been distinguished from earlier stages of the AFP (i.e., area X) by firing-rate suppression to noise stimuli (Doupe, 1997
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Figure 5. Absence of song and noise-evoked local inhibition. A, Inhibitory events are not evoked by either forward or reversed BOS playback. The cell is shown at resting membrane potential (left) and with tonic depolarizing current injection (right; recorded with a CsAc/QX-314 electrode to facilitate depolarization). Top Row, Traces show spontaneous activity including inhibitory events (arrows), visible when the cell is strongly depolarized (Vrest = +5 mV) by tonic current. Middle, Bottom Rows, Traces show that inhibitory events are not evoked during presentation of either forward or reversed BOS [median-filtered responses at rest (left) and depolarized (right); song oscillograms (BOS, middle; BOS reverse, bottom) below each trace]. For this cell, d' slightly increased after depolarization, although d' for other cells and overall decreased significantly (see Results). Conventions in A and B are as described in Figure 2E. B, Noise-evoked firing-rate suppression and membrane hyperpolarization were not accompanied by conductance changes, as measured by trains of hyperpolarizing current pulses (10 msec;
Inactivation of LMAN's local inhibitory network
The apparent lack of stimulus-evoked inhibition in LMAN was curious, because of LMAN's robust inhibitory circuitry (Livingston and Mooney, 1997
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Figure 6. The local inhibitory network within LMAN is needed to maintain a high degree of song selectivity. A, Spontaneous activity was recorded sequentially in two cells from a single bird, one before BMI application (pre) and the other afterward (post), first when BMI-induced bursting activity was evident (0.5 hr) and then after washout when bursting had ceased (2 hr; QX-314 was used in the second recording). This comparison across two cells is possible because all cells within a single bird exhibited similar temporal response profiles to song. B, Although significant responses to BOS were present only before BMI application and after washout, peaks in the averaged traces during BMI-induced disinhibition often coincided with song-induced peaks in the control condition (arrows; cells and times as described in A). C, d' measures of selectivity during and after BMI treatment are plotted as a function of Vrest (n = 10 cells; 3 spiking cells indicated with black and gray arrows; 7 QX-314 cells indicated with gray arrows only).
Inactivation of LMAN's entire local network
In several BMI-treated cells that were statistically nonselective, responses often matched the temporal profile of selective responses displayed after washout (Fig. 6B, arrows). This could reflect that highly selective extrinsic afferents to LMAN are masked by the recurrent excitation exhibited by the disinhibited circuit. To measure the selectivity of these extrinsic inputs directly, the entire local circuit was inactivated with GABA while intracellularly recording from LMAN projection neurons during song playback. GABA-induced electrophysiological inactivation is assumed to reflect the shunting of positive currents via activated Clconductances, thus preventing cells from spiking. Inhibitory interneurons were presumed to be inactivated in this way by GABA application, because they are likely to contain GABA receptors; any remaining activity would in any case be ineffective, because their postsynaptic GABAergic actions on LMAN projection neurons would be occluded by excess GABA. Inactivation of LMAN projection neurons was confirmed by passing large positive currents (+1 nA) through the recording electrode and noting that the cell failed to fire action potentials (Fig. 7B, middle; compare PSTHs in GABA with those before and after treatment; see Fig. 2B for control responses to similar currents). The spatial extent of inactivation was assessed by intracellularly staining GABA-silenced cells in disparate locations across LMAN while ejecting fixed amounts of GABA from a single location (see Fig. 7A). These measurements indicated that the entire extent of LMAN was inactivated during GABA application.
Before, during, and after GABA application, forward and reversed BOS were delivered to assess selectivity, and positive currents were applied to monitor inactivation and recovery. As GABA took effect, cells hyperpolarized slightly (VGABA,
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Figure 7. Silencing local LMAN circuitry with GABA reveals persistently song-selective extrinsic inputs. A, GABA treatment inactivated the entire extent of LMAN, as indicated by examples from two birds (cases 1, 2). All stained LMAN neurons shown were silenced by GABA application (dark spots above LMAN mark the drug bolus from the stationary pipette). B, LMAN neurons maintained selective subthreshold responses when the local circuit was inactivated with GABA. Selective subthreshold and suprathreshold responses to BOS versus reversed BOS (data not shown, but see C) are visible pre- and post-GABA. Although no spontaneous, song- or DC-evoked spiking occurred during GABA application, the same cell maintained selective subthreshold responses despite a greatly reduced amplitude of synaptic activity. Positive currents (+1 nA; 0.5 sec) injected after each playback evoked action potentials before and after GABA treatment but only small depolarizations in GABA. d' values compare BOS (depicted) with reversed BOS (data not shown) responses. The bottom row depicts the timing of song playback and current injection. C, Normalized song-evoked responses from B reveal similar shapes and time courses of BOS responses with and without local circuit activity. The two traces on the right contrast forward and reversed BOS responses in GABA (d' = 1.18). The vertical scale bar on the right (1 mV) applies to middle and right panels. Vertical dotted lines in B and C mark the onset of song-evoked response from the leftmost trace (pre-GABA). L, Lateral; M, medial; N/A, not available; R, rostral; V, ventral.
DISCUSSION
The present study shows that the LMAN neurons that directly innervate vocal premotor areas (Mooney, 1992
Intracellular staining and histological reconstruction reveal that song-selective LMAN neurons are projection neurons with spinous dendrites and bifurcated axons that innervate both area X and RA, as well as ramifying locally. LMAN projection neurons recorded in vivo differed slightly from their in vitro counterparts (Livingston and Mooney, 1997
Indeed LMAN must be intact to permit the changes in adult song structure that normally occur when the tracheosyringeal nerve is sectioned or the bird is deafened, two perturbations that create a mismatch between intended vocal output and auditory feedback (Williams and Mehta, 1999
Arguing against LMAN as a site where song selectivity arises, anatomical studies (Vates et al., 1996
Previous studies have suggested an auditory refinement role for LMAN (Maekawa and Uno, 1996
Further evidence against a refinement role for LMAN is the absence of any detectable inhibitory component in responses to either forward or reversed BOS, as might be expected if the local inhibitory network further sculpted BOS responses or suppressed responses to reversed BOS. This lack of inhibition is surprising, because electrical stimulation of DLM axons readily activates feedforward inhibition in LMAN in vitro (Livingston and Mooney, 1997
The present results do show that LMAN local circuitry may act to gate song-evoked responses to RA and area X. First, a subset (53/236) of selective LMAN neurons exhibited only subthreshold responses to song, indicating that not all LMAN neurons transmit song-selective information to their postsynaptic targets. The suppression of spiking in these neurons could reflect a local circuit influence. Second, the local inhibitory circuit is needed to maintain a high degree of selectivity in LMAN. Bicuculline treatment induced rhythmic bursting, likely because of disinhibition of a robust recurrent excitatory network formed by the local axon collaterals of LMAN projection neurons (Livingston and Mooney, 1997
FOOTNOTES Received Nov. 23, 1999; revised April 17, 2000; accepted April 17, 2000.
This research was supported by National Institutes of Health Grant R01 DC 02524 and by McKnight, Klingenstein, and Sloan Foundation awards to R.M. We thank T. Tucker, D. Feldman, D. Fitzpatrick, and members of the Mooney lab for providing thoughtful comments on previous versions of this manuscript and Dr. Jose Manuel Alonso for helpful suggestions concerning GABA inactivation.
Correspondence should be addressed to Dr. Richard Mooney, Department of Neurobiology, Box 3209, Duke University Medical Center, Durham, NC 27710. E-mail: mooney{at}neuro.duke.edu.
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